1. Field of the Invention
The present invention relates to a quinoxaline derivative, and a light-emitting element, a light-emitting device, and an electronic appliance each of which uses the quinoxaline derivative.
2. Description of the Related Art
An organic compound can take various structures compared with an inorganic compound, and it is possible to synthesize a material having various functions by appropriate molecular-design of an organic compound. Owing to these advantages, photo electronics and electronics which use a functional organic material have been attracting attention in recent years.
For example, a solar cell, a light-emitting element, an organic transistor, and the like are exemplified as an electronics device utilizing an organic compound as a functional material. These are devices taking advantage of electric properties and optical properties of the organic compound. Among them, in particular, a light-emitting element has been making remarkable development.
It is considered that the light emission mechanism of a light-emitting element is as follows: by applying a voltage between a pair of electrodes which interpose a light-emitting layer, electrons injected from a cathode and holes injected from an anode are recombined in a luminescent center of the light-emitting layer to form a molecular exciton, and energy is released to emit light when the molecular exciton relaxes to the ground state. A singlet excited state and a triplet excited state are known as excited states, and it is thought that light emission can be obtained through either of the excitation states.
In improving the element characteristics of such a light-emitting element, there are many problems which depend on the material, and in order to solve such problems, an improvement of the element structure, a development of materials, and the like have been carried out.
For example, 2,2′,2″-(1,3,5-benzenetriyl)-tris(1-phenyl-1H-benzimidazole) (abbreviation: TPBI) or the like is widely used as an electron-transporting material of the light-emitting element (see Non-Patent Document 1: X. T. TAO et al., Applied Physics Letters, vol. 77, No. 11, p. 1575 (2000)). However, a development of a material with more excellent characteristics such as further higher mobility has been demanded. In particular, in view of commercialization, reduction in power consumption is an important object, and a development of a material and a light-emitting element with more excellent characteristics has been desired.